Holey optical fibers are optical fibers including a cladding surrounding a core and having a large number of fine air holes arranged regularly or randomly along the axis thereof to reduce the refractive index of the cladding so that it has a refractive index different from the core. Holey optical fibers allow the refractive index of the cladding to be drastically changed without adding a dopant and are available in various forms, including those including a cladding having a large number of air holes arranged concentrically in a portion thereof and those including a cladding having a large number of air holes arranged over the entire region thereof. FIG. 5 shows sectional views illustrating examples of holey optical fibers. A holey optical fiber 1 in region (A) of FIG. 5 includes a core 2 and a cladding 3 surrounding the core 2, and the cladding 3 includes, in order from inside concentrically, an inner region 3a having no air holes, an intermediate region 3b having a large number of fine air holes 4, and an outer region 3c having no air holes.
FIG. 6 is a side view illustrating fusion splicing of holey optical fibers 100a and 100b. Fusion splicing is performed by melting end surfaces 105a and 105b of the optical fibers with heat generated by arc discharge between a pair of discharge electrodes 104 arranged perpendicular to the axial direction of the optical fibers 100a and 100b and bringing the end surfaces 105a and 105b into abutment. As shown in FIG. 7, however, when a holey optical fiber 100 is optically observed by simply illuminating the side surface thereof, a core 101 cannot be detected because of the presence of a large number of fine air holes 103. This prevents positioning by core alignment and therefore prevents precise fusion splicing.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2010-509641 discloses a fusion splicing method that can detect the core optically from the side thereof by filling air holes at the tips of optical fibers. In this method, the end surfaces 105a and 105b of the optical fibers 100a and 100b are heated by arc discharge at a distance from each other prior to bringing them into abutment to fill the air holes 103 at the tips of the optical fibers 100a and 100b so that the core is optically detectable. After that, the optical fibers 100a and 100b are positioned by optically detecting the core and performing core alignment, and the end surfaces 105a and 105b of the optical fibers are brought into abutment and are fusion-spliced by arc discharge heating.
FIG. 8 is a conceptual diagram illustrating the temperature profile at the tips of the optical fibers during arc discharge heating prior to bringing them into abutment. The end surfaces 105a and 105b of the holey optical fibers 100a and 100b are heated by arc discharge at a distance L from each other across an axis Y passing through the center of the pair of discharge electrodes 104. The heating temperature is highest at the position where the axis Y passes and becomes lower toward the rears of the optical fibers. Thus, the portions exposed to the highest temperature in the optical fibers are the end surfaces 105a and 105b, whose corners are rounded into a shape having a relatively large radius of curvature R. As the radius of curvature R of the corners of the end surfaces 105a and 105b becomes larger, the area fused by fusion splicing becomes smaller due to misalignment and narrowing of the spliced portion, which tends to increase splicing defects and splicing loss. Conversely, if the amount of heat generated by arc discharge is reduced for a smaller radius of curvature R, the air holes in the optical fibers are insufficiently filled, which prevents the position of the core from being detected and thus makes core alignment difficult.